In optical fibers, a supercontinuum light is formed when a collection of nonlinear processes act together upon feeding of a pump beam in order to cause spectral broadening of the original pump beam. The result may be a smooth spectral continuum spreading such as over more than an octave of wavelengths. Suitable non-linear processes are for example self-phase and cross-phase modulation, four-wave mixing, Raman gain or soliton based dynamics, interacting together to generate the supercontinuum light. In order to get the broadest continua in an optical fiber, it is most efficient to pump in the anomalous dispersion regime; however a spectral continuum may in some optical fibers be obtainable by pumping in the normal dispersion regime.
Microstructured optical fibers may be designed for supercontinuum generation due to their high non-linearity and their customizable zero dispersion wavelength. Microstructured optical fibers may be formed from a background material, e.g. silica, and comprise a solid core region surrounded by a cladding region, which comprises an array of cladding features, e.g. air filled holes, in a background matrix. The air/background material of the cladding creates an effective refractive index which may be less than the refractive index of the core region and thus permits the guidance of light within the core by a variation of the traditional mechanism of total internal reflection.
The terms “microstructured fibers” and “microstructured optical fibers” are in this context meant to cover optical fibers comprising microstructures such as photonic crystal fibers, photonic bandgap fibers, leaky channel fibers, holey fibers, etc. Unless otherwise noted the refractive index refers to the average refractive index which is usually calculated for the core and each layer surrounding it separately, whether the fiber is a standard fiber, where the core and any cladding layers surrounding that core have a substantially homogeneous refractive index, or a microstructured fiber where the core and/or one or more cladding layers comprise microstructures.
A cladding layer is defined a layer with a thickness and surrounding the core where the refractive index is substantially homogeneous or where the where the layer has a base material with substantially homogeneous refractive index and a plurality of microstructures arranged in a uniform pattern.
The zero dispersion wavelength (ZDW) is an important parameter in the generation of supercontinuum spectra where the widest spectra are produced when the pump wavelength is relatively close to the ZDW. In microstructured fibers, it is well known to shift the ZDW to thereby enable laser sources having different wavelengths to be utilised as pump sources in the generation of supercontinuum spectra.
The microstructured optical fibers will have a U-shaped variation in group index curve as a function of wavelength. The group index at a larger wavelength is matched to the group index at a shorter wavelength. A frequency-shifting soliton propagating in the anomalous-dispersion regime effectively traps blue radiation propagating with the same group index on the other arm of the “U” in a potential well and scatters the blue radiation to shorter wavelengths.
Supercontinuum generation is a complex process, and any quantitative explanation of the underlying physics must take into account a number of different fiber and pulse parameters. Nonetheless it is generally accepted that the most efficient method to obtain a very broad supercontinuum is by using a pump wavelength slightly in the anomalous group-velocity dispersion (GVD) regime of a highly nonlinear Photonic Crystal Fiber (PCF) with only one zero-dispersion wavelength (ZDW) below the absorption limit of the material. In contrast, pumping in the normal GVD regime of a PCF will in general reduce the bandwidth and require a longer length of the PCF (J. Dudley et al, “Supercontinuum generation in photonic crystal fiber”, Reviews of Modern Physics, Vol. 78, p. 1135, October-December 2006).
Typically prior art high power supercontinuum sources use a pump wavelength of around 1064 nm and a PCF with a core size of about 3.5 to 5 μm having a ZDW slightly below the pump wavelength. Typical examples of such fibers are the commercial products from SC-5.0-1040 (core size 5.0 μm, ZDW=1040 nm) and SC-3.7-975 (core size 3.7 μm, ZDW=1040 nm) from NKT Photonics. A standard calculation of the dispersion of a PCF with a given core size, shows that the ZDW decreases when the relative hole size increases (defined as hole size divided by pitch). As the core size of the PCF increases, so does the relative hole size that is required to obtain a ZDW of about 1064 nm. For very large relative hole sizes, it is possible to obtain a ZDW at 1064 nm for core sizes up to about 6 μm. Hence in order to have anomalous dispersion at a wavelength of 1064 nm in a PCF, the core size is limited to about 6 μm or less.
In supercontinuum sources it is advantageous to reduce the noise. It is an object of the invention to provide an optical fiber arranged to generate incoherent supercontinuum light with reduced noise.
The article “Zero-dispersion wavelength decreasing photonic crystal fibers for ultraviolet-extended supercontinuum generation” by Kudlinski et al., Optics Express Vol. 14, No. 12, 12 Jun. 2006, describes tapering of fibers in order to extend the generation of supercontinuum spectra from the visible into the ultraviolet. This article describes the manufacturing of tapered microstructured fibers with a regular array of microscopic air holes surrounding a solid silica core. By adjusting the drawing parameters, tapered fibers with a length in the order of 10 m are manufactured with a continuously-decreasing ZDW along their length. The article describes that this decreasing ZDW extends the generation of supercontinuum spectra from the visible into the ultraviolet. The article relates to fibers with large holes, and indicates that the shortest edge of the supercontinuum spectrum is achieved for fibers having a core size at about 2 μm. The article also states that further tapering to smaller core sizes does not provide light at shorter wavelengths, but merely decreases the power.
The article “Control of pulse-to-pulse fluctuations in visible supercontinuum”, by A. Kudlinski et al. in “Optics Express”, 20 Dec. 2010, Vol. 18, No. 26, examines fluctuations in supercontinuum systems. It is described that millimeter-long post-processed tapers result in a low spectral power density in the visible spectrum, which is detrimental to many applications. Thus, short tapers are unsuitable for incoherent SC sources.
The article thus describes tower tapered fibers. The power spectra and pulse-to-pulse fluctuation spectra of the supercontinuum spectrum are compared for a system with a 15 m uniform PCF and system with a fiber having an 8 m uniform PCF followed by a 7 meter long tapered section. It is seen that the tapered system extends the light into ultra-violet, as described earlier in the previously mentioned paper from Kudlinski, 2006. For the 15 m uniform fiber, it was found that the pulse-to-pulse fluctuations have a nearly stable level from 700 nm to 1400 nm, but that the fluctuations are increased for wavelengths below 700 nm. For the system using the tapered fiber, the pulse-to-pulse fluctuations from 700 nm to 1400 nm are similar to the system with the uniform fiber, whilst the flat level is maintained all the way down to around 400 nm for the system using the tapered fiber.
The review article “Blue extension of optical fibre supercontinuum generation” by J. C. Travers in Journal of Optics, J. Opt. 12 (2010), 113001, describes how to design fibers for obtaining the shortest possible blue edge by choosing the dispersion of a fiber, its nonlinear properties and the effective area of the fiber.
It is shown that the low wavelength edge for the supercontinuum is obtained where there is a group velocity match to the wavelength at the infrared loss edge. Furthermore the low wavelength edge is mapped as a function of pitch and relative hole size in the PCF. It is found that the shortest low wavelength edge is obtained for very large relative hole sizes (d/Λ>0.85) and at pitches around 2.0 μm. Furthermore, the pitch giving the lowest wavelength edge increases as the relative hole size decreases, to e.g. around Λ=2.3 μm at d/Λ=0.60.
These conclusions are confirmed in the review paper “Optimum PCF tapers for blue-enhanced supercontinuum sources” by U. Møller et al, Optical Fiber Technology 18, 2012, pages 304-314, wherein it is described that tapering of photonic crystal fibers has proven to be an effective way of blue shifting the dispersive wavelength edge of a supercontinuum spectrum down in the deep-blue. This paper also describes how high-energy solitons reaching the infrared loss edge through trapped and group-velocity matched dispersive waves is an effective way of blue shifting the blue edge of a supercontinuum spectrum.
The paper “Low noise wavelength conversion of femtosecond pulses with dispersion micro-managed holey fibers” by Fei Lu and Wayne H. Knox, Opt. Express, Vol. 13, No. 20, page 8172 (2005), describes how to minimize the noise for coherent supercontinuum sources, pumped with 100 femto-second pulses at 920 nm. The supercontinuum and noise spectra are compared for an 80 cm uniform fiber, a fiber having a 2.6 cm taper and a dispersion micro-managed fiber comprising a taper with length of <1 cm. It is shown that the noise is largest for the uniform fiber, and lowest for the dispersion micro-managed fiber. It is explained that this is due to the short length of the dispersion micro-managed fiber matching the length scale for the soliton fission process while being too short to allow for additional “messy fission collisions” (sic) to take place. It is worth noticing that for coherent sources the supercontinuum spectrum is seeded by the pulse itself by soliton fission processes, whereas for incoherent supercontinuum sources the supercontinuum is seeded from noise, and thus it is not possible to make a dispersion micro managed fiber as taught by Lu and Knox for incoherent supercontinuum sources.